This invention relates to a powder compaction method. The powder compacts obtained by such a method are subjected to a heat treatment or sintering process, by which various kinds of parts such as machine parts are produced.
The following methods have been known as powder compaction methods: (1) the die pressing method, in which a powder packed in a cylindrical die is compressed with punches; (2) the cold isostatic pressing (CIP) method, in which a powder packed in a rubber mold is compressed by means of liquid; (3) the rubber isostatic pressing (RIP) method disclosed in U.S. Pat. No. 5,250,255, in which a die is loaded with a rubber mold filled with a powder, and then compressed with punches; (4) the extrusion molding method, in which a slurry of powder is extruded from the die; and (5) the metal injection molding (MIM) or powder injection molding (PIM) method, in which a mixture of a powder and binder is heated until the binder has fluidity, and then injection molded. The metal injection molding together with powder injection molding are hereinafter referred to as xe2x80x9cthe PIM methodxe2x80x9d.
The die pressing method cannot be applied to compaction of shapes that are impossible to be pulled out of the die, and also pressing long shapes such as tubes is difficult with this method. The CIP method has problems in near-net-shape performance and productivity. The RIP method is advantageous over those two methods above in terms of flexibility of shapes and productivity, in that RIP permits a wider selection of shapes and provides good near-net-shape performance and productivity. However, the RIP method is less flexible than the PIM method in terms of the selection of shapes. The disadvantage of the PIM method is that it uses a mixture of a powder and binder in which the quantity of the binder reaches as much as 40 to 50 volume %. It takes a long time to eliminate such a large amount of binder from the compact in the xe2x80x9cdebinder process,xe2x80x9d and this makes it difficult to produce thick and large parts. Carbon contamination during the debinder process is also one of the problems of the PIM method.
This invention is intended to solve the problems described above that the conventional powder compaction methods suffer, by providing, first, a process in which a mold is filled with a powder by air tapping, while the powder particles are bound with each other without mechanical compaction or the application of force from outside the mold to form a compact, and then the compact is taken out from the mold, second, a process in which a binder is added to the powder so that the powder particles are bound with each other by the binder to form a compact, third, a process in which a lubricant is added to the powder, and fourth, a process in which the powder particles are bound with each other by means of heating.